使用 Semaphor、Mutex 和 PThread 的多线程

Question

我和一个朋友目前正在为大学编写基本的多线程示例。我们应该用多线程缓冲区解决 producer/consumer 问题。我们有一个使用互斥量和条件变量的工作版本，但是试图解决这个 使用信号量和互斥量遇到了三个主要问题。

问题 1：如果我们先启动消费者，他有时会随机消费一个无效的字符并崩溃。

问题2：如果我们先启动producer，有时他会等到consumer启动后才会产生任何字符，这会导致问题1.

问题 3： 我们的生产者不会填满整个缓冲区，无论有多少生产者，每次插入缓冲区后消费者都会消费。

根据我们给定的伪代码示例，至少问题 2 和 3 不应该存在。非常感谢任何答案，因为我目前找不到错误。

消费者：

void *consumer_sem(void *args) {
    printf("cons started\n");
    char c;

    while (cons_running) {
        sem_wait(occupied);
        pthread_mutex_lock(&mutex);

        c = consume();

        pthread_mutex_unlock(&mutex);
        sem_post(free);

        printf("consumer consumed %c\n\n", c);
        sleep(2);
    }   
}

制作人：

void *producer1_sem(void *args) {    
    printf("prod1 started\n");    
    char c;    
    int index=0;

    while (prod1_running) {    
        c = lowercase[index];    
        index=next(index);    

        sem_wait(free);
        pthread_mutex_lock(&mutex);

        add(c);

        pthread_mutex_unlock(&mutex);    
        sem_post(occupied);

        printf("producer1 produced something!\n");    
        printf("%d elements in buffer\n\n",getElemsInBuffer());   
        sleep(3);    
    }    
}

主要内容：

sem_t *occupied, *free;

pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;

int main(void) {
    occupied=sem_open("/occupied", O_CREAT, 0644, 0);
    free=sem_open("/free", O_CREAT, 0644, BUFFER_SIZE);

    //some unrelated code called

    pthread_create(&thread_ids[0], NULL, producer1_sem, NULL);
    pthread_create(&thread_ids[1], NULL, producer2_sem, NULL);
    pthread_create(&thread_ids[2], NULL, consumer_cond, NULL);

}

Answer 1

函数 sem_open 创建全局（named）信号量，该信号量在调用 sem_unlink() 之前一直存在。

当您运行您的程序第二次（以及更多）时，sem_open 重用已经存在的信号量，并且它的值不会被重置。您可以轻松检测到：

// With O_EXCL opening already existed semaphore will fail.
occupied=sem_open("/occupied", O_CREAT | O_EXCL, 0644, 0);
if(occupied == SEM_FAILED) {
    perror("Failed to create new semaphore");
    exit(1);
}

实际上，当一个信号量只被单进程（但多线程）使用时，用sem_init初始化就足够了：

sem_t occupied;
//...
int main()
{
    sem_init(&occupied, 0 /* do not share between processes*/, 0 /* initial value*/);
    //...
}

或者，您可以在尝试创建新信号量之前销毁旧信号量：

// Destroy the semaphore if it exists.
sem_unlink("/occupied");
// Create the semaphore again.
occupied = sem_open("/occupied", O_CREAT | O_EXCL, 0644, 0);

Answer 2

我设置并运行（通过 make 文件）发布的代码；

这是我使用的 makefile.mak，我通过 make -f makefile.mak

运行

SHELL = /bin/sh

#
# macro of all *.c files
# (NOTE:
# (the following 'wildcard' will pick up ALL .c files
# (like FileHeader.c and FunctionHeader.c
# (which should not be part of the build
# (so be sure no unwanted .c files in directory
# (or change the extension
#
SRC := $(wildcard *.c)
#OBJ := $(SRC:.c=.o)
DEP := $(SRC:.c=.d)
#INC := $(SRC:.c=.h)




MAKE    :=  /usr/bin/make

CC      :=  /usr/bin/gcc

CP      :=  cp

MV      := mv

#LDFLAGS :=  -L/usr/local/lib


DEBUG   :=  -ggdb3

CCFLAGS :=  $(DEBUG) -Wall -Wextra -pedantic -Wconversion -std=gnu11

#CPPFLAGS += =MD

LIBS     :=

.PSEUDO: all
all:  gestore personA personB

#
# link the .o files into the executable
# using the linker flags
# -- explicit rule
#
getstore: getstore.o
    #
    # ======= $(name) Link Start =========
    $(CC) $(DEBUG) -o $@ $^ $(LIBS)
    # ======= $(name) Link Done ==========
    #

personA: personA.o
    #
    # ======= $(name) Link Start =========
    $(CC) $(DEBUG) -o $@ $^ $(LIBS)
    # ======= $(name) Link Done ==========
    #

personB: personB.o
    #
    # ======= $(name) Link Start =========
    $(CC) $(DEBUG) -o $@ $^  $(LIBS)
    # ======= $(name) Link Done ==========
    #

#
#create dependancy files -- inference rule
# list makefile.mak as dependancy so changing makfile forces rebuild
#
%.d: %.c
    #
    # ========= START $< TO $@ =========
    $(CC) -M $(CPPFLAGS) $< > $@.$$$$;                      \
    sed 's,\($*\)\.o[ :]*,.o $@ : ,g' < $@.$$$$ > $@;     \
    rm -f $@.$$$$
    # ========= END $< TO $@ =========



#
# compile the .c file into .o files using the compiler flags
# -- inference rule
#
%.o: %.c %.d
    #
    # ========= START $< TO $@ =========
    $(CC) $(CCFLAGS) -c $< -o $@ -I.
    # ========= END $< TO $@ =========
    #

.PHONY: clean
clean:
    # ========== CLEANING UP ==========
    rm -f *.o
    rm -f *.d
    rm -f getstore
    rm -f personA
    rm -f personB
    # ========== DONE ==========

# include the contents of all the .d files
# note: the .d files contain:
# <filename>.o:<filename>.c plus all the dependencies for that .c file
# I.E. the #include'd header files
# wrap with ifneg... so will not rebuild *.d files when goal is 'clean'
#
ifneq "$(MAKECMDGOALS)" "clean"
-include $(DEP)
endif

使用 Semaphor、Mutex 和 PThread 的多线程

Multithreading with Semaphor, Mutex and PThread

c

linux

multithreading

semaphore

pthreads